Don Kagin holds a 98.6 ounce gold nugget in the Chroniicle studio on Friday January 14, 2011 in San Francisco, Calif. He and his business partner Fred Holabird are auctioning off the nugget in Sacramento on Wednesday March 16, 2011.

It's humongous. It's rare. It's the sort of thing prospectors used to get shot over.

It's the 98.6-ounce Washington Nugget - the largest piece of smooth California gold in existence, and by the end of today, someone at a Sacramento auction will probably have paid more than a quarter of a million dollars for it.

The nugget was dug up with a pick last March by a man strolling his land near the historic Sierra Gold Rush town of Washington (Nevada County). He promptly trotted it over to Reno to show it to mining geologist Fred Holabird, who says he promptly fell on the floor in astonishment.

"You just don't see things like that," Holabird said. "It's one in a million - no, one in a billion."

In addition to being a geologist, Holabird co-owns Holabird-Kagin Americana with coin dealer Don Kagin of Tiburon, and one of the ways they make money off historic treasures is by holding auctions. That's what they're doing today at the Sacramento Convention Center.

So far, more than a dozen people from around the world have said they will start the bidding at $250,000, Holabird said. He and Kagin took the monster nugget on a national tour of coin shows this year, from San Francisco's Old Mint building to Florida, and everywhere it went, it drew envious oohs and ahhs.

"I can't say the name of the guy who found this, because gold means a lot of things to a lot of people, if you know what I mean, and we don't want someone running over to his property to start a new gold rush or worse," Holabird said. "But I'll just say he's pretty excited. The bidding may go to $400,000."

Gold Rush history link

In troy ounces, which is what gold is weighed in, the nugget comes out to 8.2 pounds. It looks like a smooth-surfaced and lumpy chunk of, well, what it is, with veins of rock cutting through it.

If it were sold purely as metal, it would go for $137,744 at gold's current price of $1,397 an ounce, give or take a few bucks to take the rock veins into consideration. But this behemoth isn't just gold. It's tied to Gold Rush history.

During the mad, mid-1800s scramble for gold in the Sierra foothills, Washington - located about 10 miles northeast of Nevada City - was a bustling treasure hunters' town of 3,000.

Dozens of huge nuggets were found all over the mountain range during those halcyon days, the biggest being the 54-pound Magalia Nugget, scratched up in Butte County in 1859.

Increasingly rare find

Other masses of gold were found, including the 16.7-pound Fricot chunk of crystals still on display in Mariposa - but the smooth-surfaced nugget of diggings fame was a rare thing indeed.

They became even rarer in the decades that followed the Gold Rush, as all the big smooth lumps were melted down for jewelry and coins. Washington dwindled into a rustic village of about 200 people, and though a lot of gold flakes and chunks came from there, no big nuggets were ever reported.